The years go by and the boys grow into young men that never saw the sky without the turquoise-hue getting in the way. The people inside the town turned military academy turned village make a living. There is an abundance of magic inside and they can make it comfortable.
But the turquoise-hue dome stays up there even as the few adults sire new children, some girls, things stay comfortable and civilized inside.
Eventually, enough time passes (or maybe the population loses too many people) that they increase their efforts to find a way out. And they try everything, the barrier would be an admirable work of magical craft if it wasn't literally on the way of their freedom.
Today they try something new. They have been gathering energy for months now and have triple checked the theory. The practice takes them most of the day and many of the participants are drenched in sweat despite the little movement involved.
The silver-green sphere of energy folds and collapses in onto itself, creating a hole in it's surface that goes far deeper than the sphere is wide. Eventually, the sphere becomes a flat disk from some angles, but a hole when viewed from the front. People clap and cheer when the hole opens to the other side.